A principal aim of this book is to introduce upper-level students and biologists in other disciplines to this field, and to present it within its larger context. The book also suggests a useful general framework for thinking about developmental evolution. The book's organising concept is that of the genetic pathway, the sequence of requisite genetic and molecular activities that underlie a developmental process. From this perspective, the author explores the nature of the genetic, molecular, and selectional events that alter these pathways, yielding developmental change.